Method for reducing the melt flow rate of flame retardant expanded polystyrene

ABSTRACT

A method for lowering the melt flow rate of flame retardant expanded polystyrene is provided. The method comprises treating the flame retardant expanded polystyrene with an aqueous solution of sodium bicarbonate. In another aspect of the present invention a method for pelletizing recycled flame retardant expanded polystyrene is provided. The method comprises treating ground flame retardant polystyrene in an aqueous solution of sodium bicarbonate; condensing the treated polystyrene; extruding the polystyrene; and pelletizing the polystyrene. In another aspect of the present invention the electrostatic build-up on polystyrene is reduced by contacting the polystyrene with an aqueous solution of sodium bicarbonate.

This is a continuation of application Ser. No. 068,558, filed on May 27,1993 now U.S. Pat. No. 5,302,625.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to recycling expanded polystyrene,especially to the recycling of flame retardant expanded polystyrene.

2. Description of the Prior Art

Expanded polystyrene (EPS) is in wide use in applications that requireenhanced resistance to fire and heat. In these situations, which mayaccount for some 50% of all EPS use, flame retardants are frequentlyadded to the polystyrene resins to modify the combustibility propertiesof the plastic. The addition of flame retardant materials results inincreased ignition temperatures, and a reduction in burning rates andflame spread.

Flame retardant modifiers used include halogenated hydrocarbons.Typically, the flame retardant used in polystyrene resins is abrominated hydrocarbon. Flame retardants that can be used in conjunctionwith polystyrene are listed in the Modern Plastics Encyclopedia, Vol.56, No. 10A, 666-670 (October 1979), hereby incorporated by reference.Some of the brominated hydrocarbon flame retardants that can be usedwith polystyrene include: brominated alicyclic; hexabromocyclododecane;octabromodiphenyl oxide; decabromodiphenyl oxide; brominated organic;and Trisdibromopropyl antimonite.

Besides having the positive, intended effect of modifying thecombustibility properties of the polystyrene, the flame retardant alsoincreases the melt flow rate of the polystyrene. Consequently, flameretardant polystyrene flows much more readily than does standardpolystyrene. This makes it difficult, if not impossible, to recycleflame retardant polystyrene because processing equipment is typicallyset up for the melt flow rates of untreated polystyrene, not flameretardant polystyrene. In addition, the bromine in the flame retardantadditive is corrosive to the molding equipment.

Melt flow rates of polystyrene can be measured using an extrusionplastometer. The extrusion plastometer generally used in the industry tomeasure melt flow rates of polystyrene is described in ASTM D 1238,hereby incorporated by reference. During normal operation, resin ismelted in a cylinder of the instrument for a fixed period of time; aweighted piston is then permitted to push out the resin through anorifice of a specified length and diameter. Pieces of extruded resin arethen cut off at timed intervals. Using the extruded specimen weight andthe time to extrude, a flow rate is calculated. Flow rates are reportedin g/10 min.

Flame retardant expanded polystyrene typically has a melt flow ratessubstantially above that of untreated expanded polystyrene, which isreferred to throughout the specification as expanded polystyrene.

Adding further to the problem of recycling polystyrene is the fact thatflame retardant and expanded polystyrene products are often co-mingledin waste streams. These products, however, cannot be separated by sight;thus, the rheological properties of the waste stream are inconsistentand depend on the percentage of flame retardant polystyrene contained inthe waste stream. Recycling companies, as a result, have found itdifficult and expensive to recycle expanded polystyrene products from amixed waste stream and, for the most part, have simply not recycledthese products.

To recycle expanded polystyrene, the polystyrene is first ground,usually to a fine particle size, and then washed or rinsed with water.After being rinsed, the ground polystyrene is condensed by heating it inan oven to a temperature between approximately 150° C. and 200° C.Following condensing, the polystyrene is extruded into long spaghettilike strands and then pelletized. The polystyrene is pelletized bychopping up the long strands of extruded polystyrene into approximately1/16 inch pellets. Manufacturers can then use the pellets as feed stockfor an extrusion or injection molding process to manufacture newproducts.

If flame retardant polystyrene is mixed in with the original recycledmaterial, each of the resulting pellets can have different rheologicalproperties depending upon the percentage of flame retardant polystyrenemaking up each individual pellet. Thus, manufacturers would have adifficult time controlling their injection molding and extrusionprocesses because they are set up to handle melt flow rates of untreatedexpanded polystyrene and are not accustomed to handling a feed withinconsistent rheological properties.

Another problem with polystyrene products is that they tend to collectstatic electricity. This typically limits their use as packagingmaterials for electronic components and devices that are sensitive toelectrostatic discharge (ESD). When polystyrene materials are used, theelectronic components are often shielded from electrostatic discharge inmetallized bags or bags treated with an anti-static agent to prevent ESDdamage.

At present, a need exists for a method of treating flame retardantpolystyrene to reduce its melt flow rate so that it can be economicallyrecycled. Also, an inexpensive anti-static agent that can be easilyapplied to polystyrene to dissipate and prevent static build up would bedesirable.

SUMMARY OF THE INVENTION

The present invention is directed to a method for reducing the melt flowrate of flame retardant polystyrene. The method comprises treating theflame retardant polystyrene with an aqueous solution of sodiumbicarbonate (NaHCO₃). In a second embodiment of the present inventionthe flame retardant polystyrene is treated with an aqueous solutioncontaining NaHCO₃ and acetic acid (C₂ H₄ O₂).

It is believed that the process parameters for any given application ofthe invention can be readily determined by the application chemicalprocessing methods and techniques well known to those skilled in the artwithout undue experimentation. For example, if economics permit, longercontact times may be employed with less concentrated solutions. On theother hand, shorter contact times may provide sufficient melt flow ratereduction with higher concentration solutions or with recycle streamscontaining lower amounts of or finely ground flame retardant expandedpolystyrene.

From the examples it may be determined that a contact time of at leastabout 3 minutes with an aqueous solution of at least about 3 teaspoonsof sodium bicarbonate per 1/2 gallon of water adequately reduce the meltflow index of flame retardant expanded polystyrene. Since a finer grindof the feed stock exposes greater surface area, it should be apparentthat even shorter contact times and/or lower solution concentrationscould be employed to achieve the results of and are within the scope ofthe invention.

In another aspect of the present invention, a method for reducing theelectrostatic build up on polystyrene is provided. The method comprisescontacting, such as by spraying, polystyrene with an aqueous solution ofsodium bicarbonate (NaHCO₃).

Accordingly, it is an object of the present invention to provide amethod of treating flame retardant polystyrene to reduce its melt flowrate. It is a further object of the present invention to provide amethod of treating flame retardant expanded polystyrene products foundin co-mingled waste streams so that they can be recycled in conjunctionwith untreated expanded polystyrene products without separation.

The above and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionof the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

By the method according to the present embodiment of the invention,flame retardant expanded polystyrene can be made to exhibit melt flowrate properties similar to untreated expanded polystyrene. This isaccomplished by exposing the flame retardant expanded polystyrene to anaqueous solution of sodium bicarbonate. In the present embodiment theexposure is accomplished by a soaking step. It will be readily apparentto those skilled in the art that any other method for exposing the flameretardant expanded polystyrene to the sodium bicarbonate solution may beused. For example, methods such as a continuous process which providesadequate contact time are well known to those skilled in the art.

After soaking in the solution of sodium bicarbonate, the flame retardantexpanded polystyrene will burn as if it contains no flame retardantadditive. Further, the melt flow rate of the flame retardant expandedpolystyrene is reduced following treatment with the aqueous solution.

The amount the melt flow rate is reduced depends on several factors.These include soak time, grind size of the polystyrene being treated,temperature of the aqueous solution, and concentration of the sodiumbicarbonate solution.

The solution preferably contains from 3 to 5 teaspoons of granularsodium bicarbonate per one-half gallon of water. More preferably, thesolution contains approximately 5 teaspoons of sodium bicarbonate per1/2 gallon of water. In the examples below commonly available sodiumbicarbonate of a purity believed to be above 99% was used. Less puremixtures could also be used by adjusting the amount added to give asufficient sodium bicarbonate concentration to reduce the melt flowindex, provided impurities harmful to the process are not included.

Soaking time is very flexible and can be set in a range any where fromapproximately 3 minutes upwards. In one test a soak time of 3 daysprovided acceptable results. Preferably the treatment time is set in arange of approximately 5 minutes to 20 minutes, and most preferably in arange of approximately 5 to 10 minutes. As noted above it may bepossible to adjust the process parameters such that a shorter soak timemay be used.

With respect to temperature, the process according to the presentinvention generally works better at warmer than colder temperatures.Preferably the temperature of the aqueous solution is in the range of60° to 80° F. Better results are also generally obtained with finelyground polystyrene than with shaved strips or blocks of polystyrene.Typically, the finer the grind size, the better the results. This isbelieved to be because the surface area exposed to the sodiumbicarbonate solution is increased.

Following treatment with the aqueous solution, the flame retardantexpanded polystyrene preferably has a melt flow rate in the range of 12to 25 g/10 min. when tested according to ASTM D 1238-65T (cond. G). Evenmore preferably, the melt flow rate of the polystyrene is in the rangeof approximately 10 to 20 g/10 min. Most preferably, the melt flow ratefollowing treatment is approximately 10 g/10 min.

In another embodiment of the present invention, flame retardant expandedpolystyrene is soaked in an aqueous solution containing both sodiumbicarbonate and acetic acid. The source of acetic acid utilized in theexamples below was apple cider vinegar. Those skilled in the art willappreciate that other sources of acetic acid may be used. The aceticacid concentration is preferably the equivalent of approximately 2teaspoons of apple cider vinegar containing 5% acetic acid per 1/2gallon of water. The use of other acetic acid solutions providing thebenefits of the invention are also within its scope. The factorsaffecting the efficiency of the process are the same for this embodimentas for the first embodiment of the present invention.

In yet another embodiment of the present invention, the melt flow rateof flame retardant expanded polystyrene is reduced while it is in aco-mingled waste stream containing both flame retardant expandedpolystyrene and expanded polystyrene. The co-mingled waste streams,which can be treated by the method of the invention, may contain flameretardant polystyrene in any proportion. Typically commercial recyclewaste streams contain 50-60% flame retardant expanded polystyrene.However, the invention is not limited by the waste stream compositionsand waste streams containing 1%, 5%, 20%, 40%, 60%, 80% 95% and 99%flame retardant expanded polystyrene could be treated by the method ofthe invention.

The commercial waste stream is treated with an aqueous solution ofsodium bicarbonate or sodium bicarbonate and acetic acid, in accordancewith the above embodiments. Again, it is preferable for the co-mingledwaste stream to be finely ground before being treated.

In yet another embodiment of the present invention, a method forpelletizing flame retardant expanded polystyrene is also provided. Inthis embodiment, ground recycled flame retardant expanded polystyrene isexposed to an aqueous solution of sodium bicarbonate, or to a solutionof sodium bicarbonate and acetic acid, for example by soaking, to reducethe melt flow rate of the flame retardant expanded polystyrene to therange of approximately 12 to 25 g/10 min. Preferably the melt flow rateis reduced to the range of 10 to 20 g/10 min., and most preferably themelt flow rate is reduced to approximately 10 g/10 min. The groundrecycled expanded polystyrene can be treated alone or can be co-mingledwith expanded polystyrene in the feed stock to the process of thismethod.

After soaking in the aqueous solution, the flame retardant expandedpolystyrene is condensed in an oven. The treated and condensed flameretardant expanded polystyrene is then extruded into long spaghetti likestrands. Pelletized polystyrene feed stock is produced from the extrudedpolystyrene by chopping it up into short pieces, preferablyapproximately 1/16 of an inch long.

Because the melt flow rate of the pellets of recycled flame retardantpolystyrene produced by the method of the invention is comparable tothat of non-flame retardant expanded polystyrene, manufacturers usinginjection molding and extrusion processes to manufacture polystyreneproducts can use the pellets of recycled flame retardant expandedpolystyrene directly in their molding processes. Thus, the processaccording to the present embodiment of the invention provides a methodfor pelletizing recycled flame retardant polystyrene so that it can beused in processing equipment set up to process non-flame retardantpolystyrene. Furthermore, an advantage of the present process is thatflame retardant polystyrene can be treated in a co-mingled waste stream.This is important because most waste streams of EPS products reaching arecycling center contain both flame retardant polystyrene and non-flameretardant polystyrene.

In still another embodiment of the present invention, it was found thatthe electrostatic build up on ground EPS can be dissipated by sprayingthe EPS with the aqueous mixture of sodium bicarbonate or sodiumbicarbonate and acetic acid. After spraying the EPS, it was found thatit was no longer charged with static electricity. It is believed thatother polystyrene products, such as injection molded and extrudedpolystyrene products can also be sprayed with the mixture to prevent thebuild up of static electricity on these products.

The following examples are set forth for the purpose of illustrating theinvention only and are not to be construed as limitations on the presentinvention except as set forth in the appended claims.

EXAMPLE 1

An aqueous solution of sodium bicarbonate was prepared. "ARM & HAMMER®"baking soda was used for the source of sodium bicarbonate in this, andall other examples discussed in the Specification. According to The MerkIndex, 948 (8th ed. 1968), sodium bicarbonate sold in commerce is about99.8% pure. Flame retardant expanded polystyrene from a recycling centerwas then submerged in the aqueous solution and samples allowed to soakin the solution for 5 minutes, 1 day, and 3 days. The treated flameretardant polystyrene for each of the tests was ignited. From this test,it was determined that each of the treated polystyrene samples burned asif they were no longer flame retardant.

EXAMPLE 2

A 1/2 gallon aqueous solution containing 3 teaspoons of baking soda wasprepared. Both finely ground and large pieces of flame retardantexpanded polystyrene were placed into the solution to soak. Aftersoaking for 10 minutes, some of the polystyrene was removed and allowedto dry. Both the small and large pieces burned as if they were not flameretardant. The remainder of the finely ground and large pieces ofpolystyrene were removed from the solution after 20 minutes. Again, boththe finely ground and large pieces burned as if they were not fireretardant. However, with both the 10 and 20 minute treatments, thesmaller pieces burned much faster than the larger pieces.

EXAMPLE 3

A test solution comprising 1/2 gallon of water, 4 teaspoons of bakingsoda, and 2 teaspoons of apple cider vinegar was prepared. The applecider vinegar used in the experiment was manufactured by Heinz and had a5% acetic acid content. Flame retardant expanded polystyrene samplesobtained from a recycling yard were allowed to soak in the solution for10 min. and 20 min., respectively. After the 20 min. sample was removed,another teaspoon of baking soda was added to the solution to bring thetotal amount of baking soda added to 5 teaspoons. Again, flame retardantexpanded polystyrene samples were permitted to soak for 10 min. and 20min. All of the test samples burned well, like non-flame retardantexpanded polystyrene. However, the sodium bicarbonate solution appearedto work better after the fifth teaspoon of sodium bicarbonate was added.Furthermore, the flame retardant expanded polystyrene treated with theaqueous solution of baking soda and apple cider vinegar appeared to burnbetter than flame retardant expanded polystyrene treated with theaqueous solution containing just baking soda.

EXAMPLE 4

An aqueous solution containing 3 teaspoons of baking soda and 2teaspoons of apple cider vinegar in 1/2 gallon of water was prepared. A12 g mixture of expanded polystyrene was soaked in the mixture. The 12 gmixture contained 6 g of flame retardant expanded polystyrene and 6 g ofnon-flame retardant expanded polystyrene. Following the treatment, themelt flow rate of the treated mixture was tested. The results of themelt flow tests indicated that the average melt flow rate of the treatedmixture was 22.35 g/10 min. Whereas, the melt flow rate of the untreatedflame retardant expanded polystyrene was 41.05 g/10 min.

EXAMPLE 5

A 3/4 gallon test solution containing 18.11 g of baking soda wasprepared. Co-mingled mixtures of expanded polystyrene containing atleast 50% flame retardant expanded polystyrene were soaked in thesolution. After soaking, each of the co-mingled mixtures were condensedat approximately 250° F. The condensed mixtures were then melt flowtested using a TINIUS OLSEN extrusion plastometer. The test conditionsof the test were 200° C. and a 5.0 kg weight. In addition, control meltflow tests were performed on untreated flame retardant expandedpolystyrene and non-flame retardant expanded polystyrene. The results ofthe tests are listed below.

    ______________________________________                                                     Melt Flow Rate                                                   Test No.     (g/10 min.)                                                      ______________________________________                                         1           47.24                                                             2           29.69                                                             3           33.62                                                             4           37.80                                                             5           39.20                                                             6           38.81                                                             7           12.86                                                             8           28.40                                                             9           18.55                                                            10           19.97                                                            11           14.24                                                            12           13.25                                                            13           12.96                                                            14           13.40                                                            15           10.56                                                            16           12.50                                                            17           11.95                                                            18           12.25                                                            19           11.42                                                            20           11.05                                                            21           10.55                                                            22           11.38                                                            23           10.71                                                            24           10.70                                                            ______________________________________                                    

Test Nos. 1-6 correspond to the melt flow rate of untreated flameretardant expanded polystyrene test samples. Test Nos. 7-10 correspondto the melt flow rate of untreated non-flame retardant expandedpolystyrene test samples, and Test Nos. 11-24 correspond to the meltflow rates of the various co-mingled test samples treated with theaqueous solution. The melt flow rates reported for each of the tests isthe average melt flow rate calculated from at least three extruded,timed samples.

As can be seen from the table, the melt flow rate of the flame retardantexpanded polystyrene is reduced significantly after soaking in theaqueous solution of sodium bicarbonate.

Although the invention has been described with reference to preferredembodiments and specific examples, it will readily be appreciated bythose of ordinary skill in the art that many modifications andadaptations of the invention are possible without departure from thespirit and scope of the invention as claimed hereinafter. For example,while the processes according to the present invention have beendescribed in terms of a batch process, the soaking according to thepresent invention could be accomplished by a continuous flow process.Similarly soak times could likely be reduced by more finely grinding thefeed stock.

What is claimed:
 1. A method for lowering the melt flow rate of flameretardant expanded polystyrene, comprising exposing the polystyrene toan aqueous solution of an alkali metal bicarbonate for a time sufficientto reduce the melt flow rate of said flame retardant expandedpolystyrene.
 2. A method for lowering the melt flow rate of flameretardant expanded polystyrene to at or below the melt flow rate ofnon-flame retardant expanded polystyrene, comprising exposing thepolystyrene to an aqueous solution of an alkali metal bicarbonate, saidaqueous solution being of at least a minimum concentration sufficient toachieve the reduction in melt flow rate in the time of exposure.
 3. Amethod for lowering the melt flow race of flame retardant expandedpolystyrene according to claim 1, wherein said aqueous solution alsocontains acetic acid.
 4. A method for lowering the melt flow rate offlame retardant expanded polystyrene according to claim 1, wherein saidaqueous solution contains at least about 24 grams of an alkali metalbicarbonate per gallon of water.
 5. A method for lowering the melt flowrate of flame retardant expanded polystyrene according to claim 2,wherein said aqueous solution contains about 2 teaspoons of a 5% aceticacid solution per 1/2 gallon of water.
 6. A method for lowering the meltflow rate of flame retardant expanded polystyrene according to claim 3,wherein said aqueous solution also contains about 2 teaspoons of a 5%acetic acid solution per 1/2 gallon of water.
 7. A method for loweringthe melt flow rate of flame retardant expanded polystyrene according toclaim 1, wherein the melt flow rate of the polystyrene after soaking isfrom about 10 g/10 min. and 25 g/10 min.
 8. A method for lowering themelt flow rate of flame retardant expanded polystyrene according toclaim 1, wherein the polystyrene is exposed to said solution for atleast about 3 minutes.
 9. A method for lowering the melt flow rate offlame retardant expanded polystyrene according to claim 1, 1wherein thepolystyrene is exposed to said solution for a period from about 5 toabout 20 minutes.
 10. A method for lowering the melt flow rate of flameretardant expanded polystyrene according to claim 1, wherein thepolystyrene is exposed to said solution for at least about 5 minutes.11. A method for lowering the melt flow rate of flame retardant expandedpolystyrene according to claim 1, wherein the flame retardant expandedpolystyrene is co-mingled with non-flame retardant expanded polystyrene.12. A method for pelletizing a ground recycled flame retardant expandedpolystyrene feed stock, comprising:a. exposing ground recycled flameretardant polystyrene to an aqueous solution of an alkali metalbicarbonate; b. removing said exposed polystyrene from said solution; c.condensing the exposed polystyrene; c. extruding the condensedpolystyrene; and d. pelletizing the extruded polystyrene.
 13. A methodfor pelletizing recycled flame retardant expanded polystyrene accordingto claim 12, wherein said aqueous solution also contains acetic acid.14. A method for pelletizing recycled flame retardant expandedpolystyrene according to claim 12, wherein said feed stock comprisesground flame retardant polystyrene co-mingled with non-flame retardantexpanded polystyrene.
 15. A method for reducing the electrostatic buildup on polystyrene, comprising spraying the polystyrene with an aqueoussolution of an alkali metal bicarbonate.
 16. A method for reducing theelectrostatic build up on polystyrene according to claim 15, whereinsaid solution also contains acetic acid.